1. Field of the Invention
The present invention relates to a dispersion compensating apparatus that compensates waveform degradation that occurs in an optical signal due to dispersion characteristics of an optical transmission line by using an optical component that changes transmission wavelength characteristics of the optical signal with a refractive index changing member that modifies, for example, temperature or stress applied on the optical component.
2. Description of the Related Art
In recent years, the Internet is widely used, and as a result, there is a demand for higher capacity and higher speed technologies in an optical communication system. Because of the demand, a communication speed in an optical communication system has already reached 10 Gb/s at the present day, and development of a next-generation optical communication system is currently underway so that a communication speed reaches 40 Gb/s.
If the communication speed, however, reaches 40 Gb/s, wavelength dispersion of light propagating through an optical fiber can not be neglected, and for example, a waveform of signal light is distorted due to the wavelength dispersion, thereby increasing an error rate (penalty) of the signal light. Conventionally, to compensate wavelength dispersion, dispersion compensators such as a virtually imaged phased array (VIPA) dispersion compensator are used.
FIG. 15 is a schematic of a configuration of a VIPA dispersion compensator. In FIG. 15, optical signal whose dispersion is compensated is input to a condenser lens system 11 via an optical circulator 10, and then is input to a VIPA plate 12. The optical signal is reflected multiply on the VIPA plate 12. More specifically, the optical signal is output from the VIPA plate 12 so that the optical signal is reflected in different directions in accordance with each wavelength thereof. The optical signal thus output from the VIPA plate 12 reaches a free-form surface mirror (or free surface mirror) 14 via a focusing lens 13. The free-form surface mirror 14 reflects the optical signal and the optical signal enters again into the VIPA plate 12. At the VIPA plate 12, the number of how many times the optical signal is reflected multiply differs in accordance with the position at which the optical signal enters the VIPA plate 12. Thus, the optical signal returns to the condenser lens system 11 so that a path difference thereof differs in accordance with a wavelength thereof.
As a result, by fluctuating the reflectance characteristic of the free-form surface mirror 14 that is by altering the position at which the optical signal reflects on the free-form surface mirror 14, a desired dispersion compensation value can be obtained. For example, if the free-form surface mirror 14 reflects an entering optical signal in a concave region thereof, a dispersion compensation value is positive. If the free-form surface mirror 14 reflects the optical signal in a convex region thereof, the dispersion compensation value has a negative value.
In a VIPA dispersion compensator, transmission characteristics of an optical signal are asymmetrical with respect to the loss axis of the optical signal. Therefore, a transmission center wavelength differs little by little in accordance with dispersion compensation values. As a result, in the VIPA dispersion compensator, a transmission wavelength needs to be adjusted for each dispersion compensation value. In Japanese Patent Application Laid-open No. 2005-77969, a method for adjusting a transmission wavelength by adjusting a temperature of the VIPA plate 12, each time the dispersion compensation value is altered, to alter the refractive index of the VIPA plate 12 is applied. In Japanese Patent Application Laid-open No. 2001-99709, a fiber bragg grating (FBG) is used as a dispersion compensator other than a VIPA dispersion compensator.
In the conventional technologies, wavelength dispersion that occurs in an optical signal cannot be efficiently compensated.
More specifically, if the transmission characteristic of the optical signal is asymmetrical with respect to the loss axis as in a VIPA dispersion compensator, a transmission wavelength thereof needs to be an optimal value for the optical signal (that is a wavelength so that a penalty of the optical signal is the smallest). The optimal value, however, fluctuates in accordance with a bit rate and a modulation method of the optical signal. Therefore, depending on a type of the signal light, the transmission wavelength characteristic is not necessarily optimal. As a result, wavelength dispersion thereof cannot be efficiently compensated.
In a method disclosed in Japanese Patent Application Laid-open No. 2005-77969 in which a temperature of the VIPA plate 12 is adjusted each time the dispersion compensation value is modified, it takes minutes until the transmission wavelength is stably controlled by adjusting the temperature of the VIPA plate 12. Thus, the wavelength dispersion cannot be compensated until the transmission wavelength gets stable. FIG. 16 is a diagram of a relationship between fluctuation of the wavelength and time when a temperature of the VIPA plate 12 is adjusted. As shown in FIG. 16, it will be appreciated that it takes about 5 minutes until the transmission wavelength is stably controlled by adjusting a temperature of the VIPA plate 12.